NodeJS Interview Questions & Answers

What is NodeJS?

NodeJS is a JavaScript runtime built on Google's V8 Engine that allows JavaScript to run outside the browser.

Key Characteristics

• Single-threaded
• Event-driven
• Non-blocking I/O
• Asynchronous programming model

Common Use Cases

• REST APIs
• Real-time applications
• Microservices
• Streaming services
• Chat applications

Explain the Event Loop in NodeJS

The Event Loop allows NodeJS to perform non-blocking operations even though JavaScript runs on a single thread.

Execution Flow

1. Call Stack
2. Node APIs / Libuv
3. Callback Queue
4. Event Loop

Example

console.log('Start');

setTimeout(() => {
  console.log('Timer');
}, 0);

console.log('End');

Output

Start
End
Timer

Predict the Output

console.log('A');

setTimeout(() => console.log('B'));

Promise.resolve()
  .then(() => console.log('C'));

console.log('D');

Correct Output

A
D
C
B

Explanation

• Promise callbacks enter Microtask Queue
• Microtasks execute before Callback Queue
• setTimeout callback executes last

Streams vs Buffers

Both are used for handling data, but work differently.

fs.createReadStream('large-file.txt');

fs.readFile('large-file.txt');

What are Worker Threads?

Worker Threads allow CPU-intensive tasks to run on separate threads, preventing the Event Loop from being blocked.

const { Worker } = require('worker_threads');

const worker = new Worker('./worker.js');

Common Use Cases

• Image processing
• PDF generation
• Data analysis
• Encryption

Implement a Simple Rate Limiter

Restrict a user to a fixed number of requests.

const requests = {};

function allowRequest(ip) {

  requests[ip] ??= 0;

  if (requests[ip] >= 5) {
    return false;
  }

  requests[ip]++;

  return true;
}

Interview Discussion Points

• Memory usage
• Distributed systems
• Redis implementation
• Sliding window algorithm

How Would You Scale NodeJS to 10 Million Users?

Expected Topics

• Load Balancer
• Horizontal Scaling
• Microservices
• Redis Cache
• Database Sharding
• CDN
• Message Queues
• Monitoring